1. Field of the Invention
This invention relates to control devices for variable engagement transmissions and more specifically relates to an apparatus augmenting the normal control means by providing a presettable set point for tractive effort.
2. Description of the Prior Art
Tractive effort, as associated with this invention, is concerned with the degree of engine output transmitted to the drive wheels of a vehicle. The amount of engine output may be varied by means provided by a variable engagement clutch.
The prior art devices for tractive effort control are generally concerned with varying the amount of tractive effort by adjusting the engine speed of the host vehicle. This is easily done by setting the vehicle engine speed at a given rate and usually providing an overriding linkage in order to either accelerate or decelerate the engine to accommodate variations. However, in an application where the vehicle engine remains constant, tractive effort control represents different problems.
A typical application of tractive effort control may be envisioned on vehicles such as agricultural tractors and construction equipment such as scrapers and loaders. It is often desirous to run the engine on this type of vehicle at a given set speed as the engine may have been designed to provide optimum torque, horsepower, and fuel economy at a specific speed. The engine throttle is set at a predetermined point and this usually causes the drive line -- transmission, differential, axles, etc. -- to provide a constant rotational speed to the ground engaging wheels.
A vehicle which operates at a constant engine speed usually incorporates some type of variable engagement clutch-transmission means. In a variable engagement device of this type the clutch transmits a portion of the engine output to the transmission or the vehicle drive wheels. The vehicle operator controls the percentage of clutch engagement thus controlling the progress of the vehicle. Partial clutch engagement results in the vehicle traveling at a fraction of its fully engaged capability.
When the vehicle is operating at a preset speed it will occasionally encounter situations not normal to the usual operating circumstance. For instance, an abnormal situation occurs when a self-loading elevating scraper is being loaded in dirt of a certain density and compaction and the scraper encounters a more compacted area. Another example of an abnormal situation would be a loader vehicle which is performing an excavating and loading function, including travel between the source of the load and the vehicle being loaded, and requires momentary acceleration to crowd into the excavation. On these occasions it would be highly desirous to be able to override the present vehicle speed to provide optimum vehicle performance.